The present invention relates to fuel distribution systems for gas turbine engines.
Fuel distribution systems for gas turbine engines are called upon to distribute fuel from a common distributor or manifold to a plurality of fuel injectors or nozzles positioned at various locations in the engine combustor. The fuel distribution must be effected over a wide range of engine operating conditions from light-off to maximum power, which represents a considerable variation in fuel flow rates.
Furthermore, the fuel must be distributed as uniformly as practical to the fuel injectors so that generally uniform combustion occurs in the combustor without obtaining undesirable temperature differentials therein. The required degree of uniformity of flow distribution depends on particular engine designs and conventional evaluation factors. For example, individual injector flow rates deviating up to about twenty percent from the average flow rate of the plurality of injectors are considered acceptably or generally uniform for practical utilization.
In a typical engine configuration, the fuel injectors are arrayed in a vertical plane circumferentially about the periphery of an annular combustor. Consequently, the injectors are positioned at relatively different heights and thus have correspondingly different fluid heads associated with their positions. The fuel distribution system design therefore must also take into account the maximum fluid head existing between the uppermost and lowermost positioned injectors if acceptably uniform fuel distribution to all injectors is to be achieved. This is because without compensation for the maximum fluid head, fuel will flow primarily only through the lowest injector at low fuel flow rates, since it has the lowest fluid head loss.
The maximum fluid head represents a pressure loss which is constant from light-off power to maximum power. This fluid head pressure loss is added to the pressure losses associated with the various elements of the fuel distribution system from the fuel pump to the injector for obtaining a total pressure loss which is proportional to fuel flow rate. Since the maximum fluid head pressure loss is a constant, its relative effect compared to the total pressure is more pronounced at light-off power (low fuel flow rate) and typically negligible at maximum power (high fuel flow rate).
For example, for a gas turbine engine of about 1000 shp and including fuel injectors located in a vertical plane around a 13-inch diameter circle, the maximum fluid head pressure is about 0.4 p.s.i. Total pressure losses at light-off power are typically on the order of 10 psi and at maximum power on the order of 100 psi to 1000 psi. Accordingly, the effect of the maximum fluid head is negligible at the maximum power condition (e.g. 0.4 psi/100 psi=0.004) and fuel distribution will be uniform.
Each injector fuel line should thus incorporate sufficient pressure loss to overcome this maximum fluid head differential to ensure that the lowermost injector does not receive substantially more fuel than the uppermost injector. This problem is most pronounced at low fuel rates since this maximum fluid head differential then becomes significant relative to the fuel flow driving pressure developed by the fuel pump. Thus, it is necessary to design the fuel distribution system such that the pressure losses in the injector fuel line are sufficiently relatively high at low fuel flow rates to accommodate this differential fluid head consideration, and yet are not so great at high fuel flow rates as to require unduly high fuel pressurization to overcome the higher pressure losses. It is also important that the pressure loss in the plural injector fuel lines be substantially uniform with respect to each other over the entire engine operating range to avoid unacceptable fuel flow rate maldistribution and as low as possible for maximum fuel distribution efficiency.
Heretofore, these design considerations have been met through the utilization of rather complicated and relatively expensive flow dividing and fuel metering valves to achieve acceptably uniform fuel distribution throughout the engine operating range. Since one of these valves is incorporated in each injector fuel line and a typical gas turbine engine will utilize a plurality of fuel injectors, e.g., twelve or more, these valves represent a significant expense item particularly in the case of small gas turbine engines, i.e., less than 3000 horsepower. These valves, which may be of the mechanical or fluidic type, typically operate automatically in response to fuel pressure to impose the requisite variable impedances to fuel flow, i.e., pressure losses, in the injector fuel lines calculated to achieve acceptably uniform fuel distribution throughout the engine operating range.
Furthermore, the reliability of the flow dividing and fuel metering operations of these valves may be affected by any contaminants in the fuel. Thus, these valves typically require periodic servicing and in some instances replacement.
It is accordingly an object of the present invention to provide an improved fuel distribution system for a gas turbine engine.
Another object is to provide a fuel distribution system of the above character which avoid the need for a flow dividing and metering valve in each of the injector fuel lines.
Another object of the present invention is to provide a fuel distribution system of the above character wherein acceptably uniform fuel distribution is achieved over the entire range of engine operating conditions while reducing the energy or pressure loss in the individual injector fuel lines.
Another object is to provide a fuel distribution system of the above character wherein pressure loss in the injector fuel lines is generally equalized with respect to each other over the range of engine operating conditions from light-off to maximum power.
Another object is to provide a fuel distribution system of the above character which is relatively immune to being plugged by contaminants in the fuel.
Another object is to provide a fuel distribution system of the above character which is inexpensive to implement, efficient in operating and reliable over a long service life.